Photographer Brad Goldpaint was in the right place at the right time Saturday night to see some of this: he went to Crater Lake, Oregon, and at 3:30 a.m. local time on June 17th he took this surpassingly beautiful picture of a somewhat rare event: pink aurorae!

[Click to recombinate.]

Gorgeous! And weird. The colors you see in aurorae depend mostly on what’s in the air. Literally! A solar storm is an eruption of subatomic particles launched from the Sun at high speed. These interact with the Earth’s magnetic field, which, through a complicated process, sends those little beasties down into our air. They slam into the molecules and atoms in the upper atmosphere, blasting off electrons like bullets hitting concrete and sending out shrapnel.

When electrons recombine with the atoms and molecules, a little bit of energy is released in the form of light, and the color of that light depends on what’s doing the emitting. Oxygen atoms, for example, tend to glow green and/or red. Oxygen molecules (two atoms combined, like the kind we breathe) glow blue. Nitrogen molecules can glow either red or blue. Here’s a diagram from the excellent Atmospheric Optics website:

For example, the atmosphere of the Earth thins out gradually the higher you go, and when you get to about 100 kilometers (60 miles) up, different physical processes become important. One of them is called chemiluminescence — light produced by chemical processes. This can make the upper atmosphere glow in different colors. It’s faint, and best seen from space… where we conveniently keep several astronauts. Neuroscientist and amateur video maker Alex Rivest has collected pictures of this airglow taken by astronauts and made this eerie and beautiful time lapse video:

Alex took the original astronaut pictures and enhanced them somewhat to bring out the faint airglow. You can see it in lots of pictures taken from the space station, and I’ve commented on it many times. One thing I’ve been meaning to do, though, is find out what the physical process is that’s causing the air to glow, and why it creates different colors — you can clearly see green, yellow, and red glow in many of the pictures!

The way this works is simple in general, though complicated in detail — much like everything else in the Universe! Basically, during the day, in the upper atmosphere ultraviolet light from the Sun pumps energy into oxygen molecules (called O2; two oxygen atoms bound together — this is the stuff we breathe). This energy splits the molecules apart into individual atoms, and these atoms have a little bit of extra energy — we say these atoms are in an excited state. Like a jittery person who’s had too much coffee, they want to give off this energy. They can do this in a couple of ways: they can emit light, or they can bump into other atoms and molecules and react chemically with them.

If you have an excited oxygen atom sitting in space all by its lonesome, it can either dump that energy by emitting green light or red light. Usually, it’ll emit green light in less than a second after becoming excited, and it’ll emit red light on much longer timescales, like minutes. This is important, so bear with me.

I’ve been accused of having a big head (which is literally true; finding hats that fit properly can be difficult), but even I wouldn’t have any trouble squeezing the 13 trillion kilometer (8 trillion mile) wide Necklace Nebula around my noggin:

[Click to enlarynxate.]

This Hubble image shows the so-called planetary nebula, which is the product of a dying star. Deep in the center of the ring are actually two stars circling each other. As one started to die, it puffed up, literally engulfing the other star. This spun up the larger star, and the centripetal force flung off material in a huge disk well over a light years across. As the star lost its outer layers, the much hotter inner core was exposed, flooding the gas with ultraviolet light, causing it to glow like a neon sign.

Or, more accurately, a hydrogen/oxygen/nitrogen sign, the gases highlighted in this image (shown as green, blue, and red, respectively). See the knots of pink emission in the ring? As the gas was expelled, the speed of the wind increased with time while the density decreased. This faster wind caught up with and slammed into the slower wind, creating clumps and other features. You can see how the gas appears to be streaking away from the center of the ring; that’s real, as the fast wind carves away the slower one.

You can also see faint red blobs at the upper right and lower left, well away from the ring itself; those are probably the caps of a very faint (in this image, invisible) hourglass shaped nebula. The disk prevents the wind from expanding along the equator of the system, so it blows up and down, out, creating two lobes of material. Those caps are all you can see, where the gas gets mildly compressed at it expands into the gas surrounding the star system.

If this whole thing looks a bit familiar, well, it should: it’s very similar to Supernova 1987A, which I’ve written about a bazillion times (seeing as how I studied it for six years for my PhD). In this case, the central star(s) is lower mass, so not as hot as the explosion that flash-ionized the gas around the supernova. That’s why it’s fainter, even though at 15,000 light years away it’s actually ten times closer than 87a!

I love planetary nebulae. They’re weird, and pretty, and tell us a lot about how stars similar to the Sun die. In our daily lives death is rarely beautiful, but in astronomy it almost always is.